The present invention relates to calibration, and in particular, to circuits and methods for calibrating lens displacement.
Cellular phones, personal digital assistants, and a number of portable devices now include miniature electronic cameras. Many consumers have begun to use these miniature cameras as their principle means for capturing memorable photographs. Consequently, consumers now expect these miniature cameras to provide high quality photographs such as those provided by a stand alone digital camera. Correct focal length adjustment in miniature cameras is crucial in providing high quality images in a digital camera.
Digital cameras commonly utilize auto-focus (AF). For miniature cameras, implementing AF requires reduced size and power. Stepper motors, which have been traditionally used in digital camera's for the AF feature, do not scale well for miniature cameras. Additionally, active AF systems that use structural light or ultrasonics to determine focus cannot be scaled. These systems may also consume too much power.
Several technologies are being developed for AF in miniature cell phone cameras. These include lens motion type AF, lens modification type AF, and extended depth of field AF. The lens motion type AF includes voice coil, piezoelectric, electopolymer, and micro-electro-mechanical-system (MEMS) technologies. In all these technologies, high quality images require precise alignment and motion control of the lens relative to the imager. Calibration of the drive signals according to the lens actuator's displacement may be necessary in order to get high quality images in a digital camera using AF.
Current calibration techniques require laser metrology to measure distances and correlate the lens displacement to control settings. This is a slow and costly process for manufacturing and cannot be practically implemented in the device for use in the field.
Thus, there is a need for improved lens displacement calibration. The present invention solves these and other problems by providing circuits and methods for calibrating lens displacement.